1. Field of the Invention
The present invention relates to an optical communication system in which a plurality of data transmission units are connected to a U-shaped or S-shaped one-way optical transmission line, and particularly to an optical bus network including repeaters and transceivers utilizing CSMA/CD (Carrier Sense Multiple Access/Collision Detection).
2. Description of the Related Art
In recent years, local area networks which connect computer systems and data terminals located at different points in the same building or factory have been attracting public attention as a basic means of accomplishing office automation and factory automation, and various systems have been proposed and put into a practical use. A bus type communication system using CSMA/CD is the most famous among those mentioned above, and is simple in structure and low in cost. The typical system of this type is a local area network called Ethernet which was announced after joint development by DEC, Intel and Xerox.
FIG. 1 illustrates the basic structure of this system where nodes A and B in separate locations are connected by a bus consisting of a coaxial cable. In general, several tens of nodes are connected to the same bus. In FIG. 1, only two nodes A and B are shown for simplicity. Each node A and B is composed of a transmitter TX which receives send data SD and transmits on the coaxial cable, a receiver RX which receives data flowing on the coaxial cable and sends it to the node terminal (not shown) as received data RD, a collision detection circuit D which detects collisions between transmission and reception and sends a collision detection output CD to the node terminal and a tap T which physically connects the transmitter TX and receiver RX to the coaxial cable.
FIG. 2 illustrates the operation of a communication system using CSMA/CD. Usually, each node A, B and C monitors communication between other nodes on the bus during their communication using a carrier sense technique and starts transmission only when an idle condition is confirmed. The communication burst A of the node A and the burst B of the node B shown in FIG. 2, as illustrated, are sent separately and the communication is successful. However, when transmission by one node is started after an idle condition is confirmed by that node, other nodes may sometimes start sending data at the same time. In such a situation, the send bursts collide on the bus. In the CSMA/CD system, when such collision is detected between, for example, nodes A and C, the nodes which have started transmission immediately stop and each node starts re-transmission after a certain period of time preset in accordance with a random number for each node. The send bursts A.sub.2 and C in FIG. 2 illustrate such re-transmission. A local area network such as this is capable of freely sending or receiving data between selected terminals through network or sending simultaneously data to several terminals from a single terminal.
Since the local area network shown in FIG. 1 is restricted in bandwidth because of the coaxial cable, it is difficult to send a wide band signal, it is difficult to transmit at a high speed and transmission quality is reduced by noise resulting from the mismatching of impedance at the coupling points of the nodes and the coaxial cable. As a result, an optical bus network, where the local area network uses an optical fiber cable in place of a coaxial cable, has been developed.
When sending or receiving an optical signal using the optical fiber cable used as the transmission line, a structure using one-way transmission is generally employed because of the mutual influence that transmitted and received optical signals can have on each other and because of the characteristics of optical directional couplers. When such one-way transmission is employed, the transmit and receive lines are formed in pairs to form the transmission line bus. U-shaped and S-shaped transmission lines are used as the transmission lines in such local area networks. When an optical fiber is used in the U or S shaped lines, transmission loss caused by the optical fiber itself is small, but loss in the optical signal coupling and separating apparatus for connecting data transmission unit nodes is large and, therefor, it is necessary to amplify the optical signal repeatedly by providing repeaters even for a comparatively short transmission line such as in a local area network.
FIG. 3 shows a U-shaped transmission line, as discussed above, in which resistive terminators 3 and 4 are provided at one end of the transmit line 1 and receive line 2, repeaters 5-1-5-n are provided at specified distances, the othe end of the transmit line 1 and receive line 2 are connected by return loops, and the data transmission units or transceivers (nodes) 7-1-7-m are connected to the transmit line 1 and receive line 2. Connection of data transmission units 7-1-7-m with the transmit line 1 is established through an optical directional coupler 8-a, while connection with the receive line 2 is established through an optical directional coupler 8-b.
FIGS. 5(a) and (b) respectively illustrate the operation of the optical directional couplers 8-a and 8-b. The optical directional coupler 8-a, which inserts the send signal SD of the data transmission units 7-1-7-m to the transmit line 1, is composed of a coupler film mirror 9-a which allows the signal SD.sub.1 on the transmit line 1 to directly pass back to the transmit line 1 as SD1. The transmit signal SD sent from the data transmission units 7-1 -7-m is totally reflected by the coupler film mirror 9-a and directly sent to the transmit line 1. The optical directional coupler 8-b which branches the received signal RD.sub.1 on the receive line 2 to the data transmission units 7-1 -7-m is composed of a coupler film mirror 9-b. The coupler film mirror 9-.sub.b partially passes the received signal RD.sub.1 on the receive line 2 and sends it back to the receive line 2 as the signal RD1 and also branches the signal to the data transmission units 7-1-7-m as the receive signal RD. Accordingly, the repeaters 5-1-5-n are provided in accordance with the number of optical directional couplers 8-a and 8-b, and as a result of the number of data transmission units 7-1-7-m and thereby the optical signal on the transmission line can be repeated through amplification and waveform shaping, etc.
FIG. 4 shows the S-shaped transmission line, as explained above, in which resistive terminators 13 and 14 are provided on one end of transmit line 11 and on one end of receive line 12, the other end of the transmit line 11 is connected to the other end of receive line 12 through a return loop 16, repeaters 15-1-15-n are provided at specified distances on the line pair. The three lines line 11, 12 and 16 are connected in the shape of an S (or a letter Z) and data transmission units 17-1-17-m are connected to the transmit line 11 and the receive line 12. In this S-shaped transmission line, a signal is not inserted or branched in the return loop 16 and therefore the signal need not be amplified and repeated while in the loop 16, but it is of course possible to form a structure in which the signal is amplified and repeated by the repeaters 15-1-15-n while in the loop 16. The operations of optical directional couplers 8-a and 8-b are the same as those in FIG. 5.
Explained hereunder are operations when the CSMA/CD system is employed for sending and receiving control in the optical bus network constructed as shown in FIG. 3 and FIG. 4. In the U-shaped transmission line as explained above, upon detection of a lack of data on the receive line 2 sent through the return loop 6 from the transmit line 1, data is sent to the transmit line 1 from the data transmission units 7-1-7-m. For example, if data is sent from the data transmission unit 7-1 to the data transmission unit 7-2, the data is sequentially amplified and repeated by the repeaters 5-1-5-n and is carried by the receive line 2 after it is returned by the return loop 6. The data on this receive line 2 is also amplified and repeated by the repeaters 5-1-5-n and received by the data transmission unit 7-2. The remaining signal is applied to the resistive terminator 14. The data sent from the data transmission unit 7-1 is returned by the return loop 6 attached to the transmit line 1 and is sent to the receive line 2 and then received and detected by the data transmission units 7-1-7-m. Therefore, the data transmission units 7-2-7-m determine that the other data transmission units are busy, determine whether the data is addressed itself in accordance with its address and the send data transmission unit 7-1 determines whether the data sent therefrom has been returned or not.
In the S-shaped transmission line, the data transmission units 17-1-17-m determine whether there is data on the receive line 12 and control transmission of data because the data on the transmit line 11 is returned by the return loop 16 and carried by the receive line 12.
An apparatus or terminal having an interface conforming to IEEE regulation 802.3 for for the existing coaxial network is generally connected to such optical network. The IEEE regulation 802.3 specifies access procedures and electrical characteristic of the interface. For example, a maximum delay time for the network is set to 51.2 microseconds and, as shown in FIG. 6, the data packet format includes a preamble P of 64 bits, a distant (receive) apparatus address DA of 48 bits, a self (transmit) apparatus address SA of 48 bits, data length information L of 16 bits, data DATA of N.times.8 bits and a check code CRC of 32 bits. The preamble P is composed of a pattern where "10101010" is sent seven times and finally "10101011" is sent. The time from the detection of a collision in the CSMA/CD system and sending of jam pattern of 32 bits or 48 bits after detection of collision is also specified.
The delay time of the network mentioned above is defined by the period from the sending of a signal on the transmit line 1, from the data transmission unit 7-1 in FIG. 3, to the reception of such a signal on the receive line 2 by the same apparatus. Therefore, expansion of the network and the scale of network is severely restricted. It is difficult to add data transmission units to a system using interfaces conforming to the interface procedures specified by IEEE regulation 802.3.
The repeaters 5-1-5-n of FIG. 3 are respectively provided with optical semiconductor elements such as a light sensing and emitting elements which convert an optical signal to an electrical signal and convert the signal again into an optical signal after amplification and waveform shaping. As shown in FIG. 7(a), the light emitting elements (LED, LD) suddenly increase in optical output when a drive current exceeds a predetermined threshold value I.sub.B. In an ordinary optical communication system, a drive current is controlled such that a bias current I.sub.B is applied to the light emitting element, to help control and speed up the optical output for ON-OFF control. In the optical bus network employing the CSMA/CD system, sending and receiving operations are controlled by detecting the existence of data (existence of an optical signal) on the receive line. A light emitting element can often send an erroneous optical signal because of noise, etc. especially when a bias current I.sub.B is always applied and erroneous detection of data on the receive line occurs.
Accordingly, in such an optical bus network, when the data transmission unit or repeater is not sending or receiving a signal, a bias current I.sub.B is not applied and the light emitting element is kept in the completely OFF condition. A bias current I.sub.B is applied after the light sensing element receives a signal to transmit in order to mor easily control the ON and OFF of the light emitting element. Thereby, as shown in FIG. 7(b), the time period (t.sub.2-t.sub.1) from the time t.sub.1 for receiving the preamble patter indicating the heading part of the data to the time t.sub.2 for sending the optical signal, after the transient condition of the light emitting element has passed, is long and the data received during this time period disappears. Namely, since preprocessing for electrical-to-optical conversion by the photo semiconductor element is required, the optical signal is not sent during the initial rise period. As a result, the signal is partly attenuated by each of the optical repeaters.
In a network utilizing a transmission line such as a coaxial cable, when repeaters are provided to compensate for loss in the transmission line, loss of the carried signal by the repeaters can be suppressed by as much as several bits to several ten of bits. Since loss of signal by insertion and branching thereof is small in the coaxial cable, a system can be configured using only two repeaters in the local area network. Therefore, data transmission can be realized by adding a comparatively short preamble pattern.
Using the interface procedures of IEEE regulation 802.3 in an optical network results in a disadvantage in that, since the preamble pattern is specified as 64 bits, not only the preamble P but also addresses DA and SA can be lost by attenuation of the signal by the optical repeaters. In such a situation, communication is no longer possible.
Because the optical network has only one direction of communication from the point of view of control, construction, cost and reliability, the U-shaped or S-shaped networks are used. When a network is formed using an optical transmission line, the transmitting signal is returned by the return loop of the U-shaped or S-shaped transmission line and received and detected to determine whether transmission has occurred. Unlike the coaxial cable network, the fact of transmission cannot be detected immediately. Accordingly, when a device having an interface specified by IEEE regulation 802.3 is connected to an optical network, it does not conform to the regulation and some modification is required of the device for the connection to the optical network.
In a CSMA/CD optical system, such as illustrated in FIG. 4, data transmission is started after absence of data on the receive line 2 is detected. The data transmission unit 7-m nearest the return loop 6 has a high probability of acquiring the right of transmission, resulting in a transmission priority in accordance with transceiver connecting position on the bus and preventing construction of a network having equal access priority for all transceivers. To prevent such an inherent transmission priority, it has been proposed that the timing associated with detecting the data on the receive line 2 in the data transmission units 7-1-7-m be preset corresponding to the transmission delay time. This system, however, has a disadvantage in that it allows the maximum transmission delay time to become large in accordance increases in the scale of the network making the efficiency of the network low.
In the S-shaped transmission line shown in FIG. 4, for example, the time required for transmitting the data sent from the data transmission unit 17-1 nearest the resistive terminator 13 to the receive line 12 through the return loop 16 so that it is detected by the data transmission unit 17-1 is almost equal to the time required by the data sent by the data transmission unit 17-m nearest the resistive terminator 14 for transmission over the receive line 12 after being carried by the return loop 16 to be detected by the data transmission unit 17-m and, therefore, the transmission priority due to the connecting position of the data transmission unit does not result.
However, this S-shaped transmission line has a disadvantage in that since the return loop 16, which is the same length as the transmitting line 11 or receive line 12, is required, total transmission delay time becomes very large, detection of data on the transmission line is also delayed and the probability of creating a collision between transmitted data becomes high.